A series of experiments on the early development of auditory perception is continued. The primary purpose of this research is to examine the development of frequency selectivity. Frequency selectivity is a measure of how well the auditory system can separate different tones; or conversely, how much tones that differ slightly in frequency are perceived as similar. The results will document the normal development of the ability to resolve different frequencies. Preliminary methodological studies will improve procedures to measure the perceptual abilities of newborns. These techniques involve measuring the natural response of newborns to suppress their vocalizations when they hear an acoustic stimulus. These experiments will optimize the measurement of changing response rates, exploit the constantly changing attentional states of newborns, and determine how duration affects thresholds. In the core of the proposal are psychophysical experiments designed to document the development of frequency selectivity. Four different auditory masking paradigms (critical ratios, critical bands, notched noises and masking patterns) are used in behavioral experiments to measure different aspects of frequency selectivity in the developing auditory system. Tests of difference limens assess improvement in the ability to detect subtle changes in the acoustic stimuli. The immediate significance of the research is in thorough documentation of neonatal auditory development and in improved psychophysical techniques for young subjects. This understanding of normal auditory development will aid in the diagnosis and treatment of communicative and perceptual disorders.